Hydrogen is abundant in the universe but scarce on Earth, as it is locked in compounds or escapes from the upper atmosphere, due to boil-off. It is produced mainly through water-dependent processes like electrolysis and steam reforming, with water also forming as a byproduct in its use.
The three main isotopes of hydrogen are protium, the most common form with no neutron; deuterium, which has one neutron and is stable; and tritium, which has two neutrons and is radioactive. In addition to these, there are a few heavier unstable isotopes of hydrogen which exist briefly before decaying.